About electronic equipment such as laptop computers, reproduction equipment for video and music, and game machines as well as mobile phones and handheld information terminals, scenes to carry increase. It is necessary for these electronic equipment to ensure reliability in various use environments, and component packaging technology with high strength to vibration at the time of carrying, a load at the time of drop impact, or the like is demanded. On the other hand, development of high density packaging technology and thin packaging technology is strongly demanded because the number of components packaged in equipment increases to enhance functionality. In order to solve problems of densification and slim-thickness and to solve a contrary problem of an increase in strength, wide-ranging proposals about densification and slim-thickness have been made.
Among these, Japanese Patent Laid-Open No. 2003-62945 (Patent document 1) discloses a multilayer-wired substrate in which a fabric and a nonwoven fabric are used for the base of an insulating layer to ensure the strength of a printed board on which electronic components are mounted in the insulating layer of the substrate. Further, Japanese Patent Laid-Open No. 2004-263112 (Patent document 2) discloses a technology for reducing the thickness of an interlayer insulating layer configuring a multilayer-wired substrate.
Since the electronic equipment is mobile equipment, fitness with people is also strongly required. Because of this, a shape design of an equipment case is invented. In equipment like this, a substrate with a flexible structure, which is called a flexible wiring board, may be used in which a polyimide is mainly used for an insulating layer, as disclosed in Japanese Patent Laid-Open No. 2000-151047 (Patent document 3).
When mobile electronic equipment is designed so as to achieve the fitness with people, it is supposed that an equipment case is rounded or is formed in a cylindrical shape to put it on a wrist. In the inventions according to Patent documents 1 and 2, it is not thought that the multilayer wired substrate can be bent without causing damage when the substrate is applied to a curved surface of the above equipment case.
Patent documents 1 and 2 disclose that a material obtained by impregnating glass cloth or a nonwoven fabric with a resin for curing is used for respective insulating layers of the multilayer substrate. A mechanical characteristic of an in-plane direction component shows an isotropy in the material obtained by impregnating the glass cloth or the nonwoven fabric with resin for curing. Since this isotropic material is used for all interlayer insulating layers, the multilayer substrate is difficult to be bent and it has very high rigidity. In Addition, if the substrate becomes when it is fitted to a curved shape, it is possible that the insulating layer will crack which will cause the interlayer to lose its insulating properties. Accordingly, the multilayer-wired substrates according to Patent documents 1 and 2 are supposed to be used in a plane form. It is not thought that multilayer-wired substrates can be are used when their substrate surfaces are bent (curving the substrates).
In order to form a substrate into a curved shape as described above, a thin substrate has an advantage because of a circumferential difference between a concave side and a convex side of the curved substrate. However, the substrate itself must be rigid to ensure that it can be used for mobile electronic equipment. Compared with a multilayer rigid substrate, a substrate obtained by multi-layering of a flexible wiring board as shown in Patent document 3 (that is, a multilayer flexible substrate) uses a polyimide resin material for an insulating layer, and therefore, there is a problem of low rigidity in the substrate itself. Additionally, regarding the polyimide, there is also a problem that the cost of the material is high.